1. Field of the Invention
The present invention relates to vocoding voice and data in a mobile communication system and, more particularly, to a device and method for scrambling/descrambling voice and data for a mobile communication system whereby communication signals on a cable channel can be scrambled and descrambled irrespective of the operational mode of a system vocoder in a digital mobile communication system during voice data transmission between mobile stations with the vocoder.
2. Description of the Related Art
Generally, voice is transmitted after compression in a digital mobile communication system in order to support a large number of mobile stations within a limited capacity of radio channels. For voice compression, there is used a voice scrambling algorithm, normally, a vocoding algorithm in the field of the mobile communication system because the vocoding algorithm is characterized by low data rate and voice quality for the cable network. The vocoding algorithm, which is an error-containing algorithm, produces more errors between the restored voice and the original one with an increase in the number of iterative compression and restorations. That is, the tone quality of the restored voice deteriorates.
FIG. 1 is a diagram for explaining interval-based transmission of voice and data between mobile stations in a general mobile communication system.
As illustrated in the figure, when an originating mobile station 1 compresses voice and transmits the compressed voice to a transport station 2 in the form of packet, then a vocoder (not shown) in the transport station 2 restores the packet to data in the PCM (Pulse Code Modulation) form. The PCM data are sent to a transport station 2′ serving a target mobile station 1′ via an exchange 3 and then recompressed at the vocoder provided in the transport station 2′. Here, the transport station 2 includes a base station controller and a base station. The compressed packet is sent to the target mobile station 1′ via a radio channel.
The mobile station 1′ restores the received packet with an internal vocoder. Through these procedures, the mobile station can receive voice from the originating mobile station.
Finally, the voice undergoes two vocoding steps during transmission between the two mobile stations. In this event, the voice packet compressed and transmitted from the originating mobile station 1 via a radio channel is packet-bypassed at the system vocoder, thereby reducing the number of vocoding steps and decreasing deterioration of tone quality. This packet bypass process is also utilized in data transmission. Here, between the mobile station 1 or 1′ and the transport station 2 or 2′ is the radio channel interval and between the transport station 2 or 2′ and the exchange 3 is the cable channel interval.
Now, a description will be given to a device for vocoding voice signals according to prior art with reference to FIG. 2.
FIG. 2 presents the patterns of a packet bypassed at the vocoder in a general mobile communication system, wherein FIG. 2a shows the format of the bypassed voice packet and FIG. 2b shows the format of the bypassed data packet. FIG. 3 is a schematic block diagram of a vocoding device in the respective operational modes in a mobile communication system according to prior art.
First, the vocoding device in the mobile communication system includes a vocoder 20 and an upper processor 10. The upper processor 10 includes the entire of the transport stations 2 and 2′ and the exchange 3, except the vocoder 20.
The vocoder 20 includes first to fourth mode switching blocks 22, 26, 27 and 31 and first to sixth vocoding processors 23, 24, 25, 28, 29 and 30 for performing a mode-based vocoding.
The above-configured vocoding device in the mobile communication system according to the prior art operates in three operational modes during data transmission between the mobile stations.
The first operational mode is enabled when the vocoders in the mobile stations are distinct, for example, 8 Kbps EVCR (Enhanced Variable Code Rate) and 13 Kbps QCELP (Qualcomm Code Excited Linear Predictive) vocoders, in which mode the system vocoders perform a normal vocoding process, i.e., voice scrambling and descrambling. In this event, the pattern of the transmission data is packet data on the radio channel (i.e., a transmission line between the mobile station and the system vocoder), and PCM data on the cable channel (i.e., a transmission line between the system vocoders between which the exchange is interposed).
The second operational mode is enabled when the vocoders in the mobile stations are identical, for example, EVCT vocoders or QCLP vocoders, in which case the system vocoders operate in a voice packet bypass mode. In this event, the pattern of the transmission data is packet data on the radio channel, and bypassed voice packet data as shown in FIG. 2a on the cable channel.
The third operational mode is enabled during data transmission between the mobile stations, in which mode the system vocoders operate in a data packet bypass mode. In this event, the pattern of the transmission data is packet data on the radio channel, and bypassed data packet as shown in FIG. 2b on the cable channel.
The conventional vocoding system operating in the above-stated three operational modes is illustrated in FIG. 3.
As show in FIG. 3, the vocoder 20 receives mode information “E” from the upper processor 10 and operates in the corresponding mode under the control of the mode controller 21.
For example, when the vocoder 20 receives the first mode information from the upper processor 10, the mode controller 21 switches the switching blocks 22 and 26 to the first vocoding processor 23, which vocodes voice packet data transferred from the originating mobile station via a radio channel “A” into PCM data and sends the PCM data to the exchange via a cable channel “C”. The mode controller 21 also switches the switching blocks 27 and 31 to the fourth vocoding processor 28, which vocodes the PCM data transferred from the exchange via a cable channel “D” into packet data and sends the packet data to the target mobile station via a radio channel “B”.
If the vocoder 20 receives the second mode information from the upper processor 10, the mode controller 21 switches the switching blocks 22 and 26 to the second vocoding processor 24, which bypasses voice packet data transferred from the originating mobile station via the radio channel “A” and sends the bypassed voice packet data to the exchange via the cable channel “C”. The mode controller 21 also switches the switching blocks 27 and 31 to the fifth vocoding processor 29, which bypasses voice packet data transferred from the exchange via the cable channel “D” and sends the bypassed voice packet data to the target mobile station via the radio channel “B”.
When the vocoder 20 receives the third mode information from the upper processor 10, the mode controller 21 switches the switching blocks 22 and 26 to the third vocoding processor 25, which bypasses data packet transferred from the originating mobile station via the radio channel “A” and sends the bypassed data packet to the exchange via the cable channel “C”. The mode controller 21 also switches the switching blocks 27 and 31 to the sixth vocoding processor 30, which bypasses data packet transferred from the exchange via the cable channel “D” and sends the bypassed data packet to the target mobile station via the radio channel “B”.
As such, the vocoder 20 communicates packets on the radio channels “A” and “B” as well as mode information with the upper processor 10 to perform an adequate processing based on the mode information, and communicates bypassed packet data and PCM data as shown in FIG. 2 on the cable channels “C” and “D” based on the operational mode.
The upper processor 10 not only controls the vocoder 20 but also enables packet communication between the mobile station and the system vocoder through the radio channels.
However, the vocoding system according to the prior art involves a problem in that the voice data on the cable channels are susceptible to overhearing while the system vocoders is operating in the respective modes.
For example, in the first operational mode, i.e., normal vocoding mode, the PCM data on the cable channels are ready to be overheard by an overhearing instrument.
In the second operational mode, i.e., voice packet bypass mode, the bypassed voice packet data are not easy to overhear by a general PCM overhearing method but susceptible to overhearing, because the voice packets are positioned in the predefined fields in a given order, as shown in FIG. 2a, with a limited number of cases for finding a specific voice packet among the entire bypassed voice packets at a position.
In the third operational mode, i.e., data packet bypass mode, the data packets can be extracted simply by eliminating a flag field having the same pattern, because the bypassed data packets iteratively appear on the cable channels, as shown in FIG. 2b. 